A contactless switch tends to be unstable immediately after the power is turned on because of the instability in the circuit voltage. In particular, in a proximity switch of high frequency oscillation type, because a normal detection of an object is possible only after the oscillation circuit has started a normal oscillation, it is necessary to suspend the output of the power source for a certain time interval after the power is turned on.
Therefore, a contactless switch is generally equipped with a power source reset circuit which permits an output from the contactless switch only after a certain time interval from the time the power is turned on and the circuit voltage has reached a certain level. There is known such a reset circuit in which a capacitor for integration is charged by a very small current and the output of the output is delayed by the time interval until the charge voltage has reached a certain level. According to such a reset circuit, the delay time is determined by the capacitance of the capacitor for integration.
In a contactless switch which is required to be small in size, a large capacitor can not be used and a capacitor of, for instance, no more than tens of pF's is usable. Therefore, conventionally, a Miller circuit of a Darlington connection for a long delay time has been used for charging the capacitor.
FIGS. 7 and 8 are diagrams showing such power source reset circuits based on Miller circuits. For instance, according to the prior art shown in FIG. 7, a constant current source 1 with an output in the order of micro A's is used along with a pair of NPN transistors 2 and 3 in Darlington connection, and a capacitor for integration C is connected across the collector and the base of the transistor 2 so that the equivalent or effective capacitance of the capacitor for integration C would be the capacitance of the capacitor multiplied by the current gain (hfe) of the two transistors and the voltage across the capacitor C is detected by a comparator 4 so as to obtain a desired delay time. FIG. 8 likewise shows a Miller circuit formed with an NPN transistor 5 and a PNP transistor 6 in Darlington connection.
According to such a conventional power source reset circuit, the delay time may be extended because the equivalent capacitance of the capacitor C is equal to the capacitance times the hfe of the two Darlington transistors. However, because the current gain of a transistor varies depending on the temperature, the temperature fluctuation is also multiplied by the current gain. Therefore, the delay time tends to fluctuate a great deal according to the temperature, causing the problem that the reset time upon turning on power tends to be unstable.
There are control methods in which the power source applied to a contactless switch is frequently disconnected and, in such a case, if the power is turned on while the capacitor for integration retains some electric charge therein, the power source reset circuit may not be able to operate properly, sometimes starting its action and producing an erroneous output. According to conventional power source reset circuits, no provision was made to assure that the contactless switch may be activated properly upon turning on of the power and erroneous action was possible when they were used in applications where the power sources are frequently disconnected.